Various aspects of Magnetic Random Access Memory (MRAM) devices are described for example in U.S. Pat. No. 5,640,343, U.S. Pat. No. 6,081,445, U.S. Pat. No. 6,667,899, U.S. Pat. No. 6,430,084, U.S. Pat. No. 6,853,599, U.S. Pat. No. 6,815,783, U.S. Pat. No. 6,781,871, and U.S. Pat. No. 6,778,428, each herein incorporated by reference.
An exemplary known MRAM device includes a magnetic tunnel junction (MTJ) having two layers of magnetic material that sandwich a non-magnetic layer. A first one of the magnetic layers, which can be referred to at the “pinned” layer, has a fixed magnetic orientation, and the other magnetic layer, which can be referred to as the “free” layer, has a changeable magnetic orientation. An electrical resistance across the layers of the MTJ varies depending on the magnetic orientation of the free layer with respect to the pinned layer. When the magnetic orientations are parallel (geometrically parallel and pointing in the same direction), the resistance is lower than when the magnetic orientations are anti-parallel (geometrically parallel and pointing in opposite directions). Subjecting the free layer to a magnetic field above a strength threshold can cause the magnetic orientation of the free layer to change, and remain changed after the magnetic field is removed until another magnetic field is applied to change the magnetic orientation of the free layer back to what it was.
U.S. Pat. No. 5,640,343 describes, for example, an arrangement where an array of MTJs is placed between electrically conductive lines so that each line on a first side of the array passes over a column of MTJs, and each line on the second, opposite side of the array passes over a row of the MTJs. Thus the array is sandwiched between sets of parallel conductive lines where the lines on one side of the sandwich are perpendicular to the lines on the other side of the sandwich. When a current is passed through one of the lines, it produces a magnetic field that influences each of the MTJs underneath it. The array is configured so that the magnetic field from only one line is not sufficient to change the magnetic orientation of MTJs underneath it, but the combined magnetic field experienced by an MTJ when the lines on both sides of it are activated is sufficient to change the magnetic orientation of that MTJ's free layer (when the fields are opposite to an initial magnetic orientation of the free layer). Thus passing a current through one of the column lines and one of the row lines switches only one MTJ, the MTJ sandwiched between both the column line and the row line at the intersection or crossover region of the column line and the row line. The state of the MTJ (e.g. high or low resistance) can be discerned or read, for example, by applying either a voltage or a current across the MTJ and monitoring the corresponding current or voltage.
Lines to provide magnetic fields to switch one or more MTJs can be bulky and magnetic fields applied to switch an MTJ can adversely affect neighboring MTJs, particularly as size of MTJs and/or MRAM devices containing MTJs is reduced.